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Answers

The last section would worry me, "Unmonitored VLCDs with insufficient macronutrient and mineral intake have the potential to cause sudden death."

A Lg order of french fries from McDonalds is about 500 calories, so, you're running on somewhere between a single bag of fries worth of energy and less than two bags worth, for an entire day. You'll need to optimize the nutrition or suffer some serious consequences (like ventricular tachycardia and fibrillation.)

If you've been able to keep this up for weeks / months, I would imagine thyroid function is already done (cold / nauseous) and you're starting to feel the "tired but wired" sensation of extra adrenaline without food / fuel.

For me, 1500 feels pretty low and 2500 feels very high. 1800-2200 kcal is very comfortable for my lifestyle, especially cycling from low to high depending on whether I'm active and hungry that day or not, based on intermittent fasting. 1000 and 3000 for me are out of range, where 550 kcal on a daily average is through the floor into the red danger zone. I see a quote from a doctor Hernried at a weight loss clinic in California (OTC Medical Weight Loss Group) that says "No matter how good the nutrition is in the product, the brain will start to break down under 600 calories."

Maybe you're looking at too small of a timeframe and missing some hidden calories? At that point, I would be drinking fruit juices and just getting the calories in and working to get my overall energy levels higher with daily activity. As others have mentioned, you should get to a Doc.

Low carb or very low carb? Can give you constipation or hard poop. Up your fluid intake, and either take supplementary magnesium to "get things moving", or maybe some fruit? Could be the reason. But see a doctor just in case though!